The present invention relates to a spacer which is disposed in a coaxial loudspeaker for mounting a speaker unit for a high audio frequency range and a middle audio frequency range.
A coaxial loudspeaker is a complex speaker comprising coaxially mounted two or more speaker units housed in a single frame, thereby providing a multiway loudspeaker. The coaxial speaker is compact in size so as to be preferably mounted on an automobile.
Referring to FIG. 17, a two-way coaxial loudspeaker comprises a woofer 9 and a mid-and high-range speaker unit 11 coaxially mounted on the woofer 9. The woofer 9 has a pole yoke 1 having an integral center pole 2, an annular magnet 3 mounted on the yoke 1, and an annular plate 4 mounted on the magnet 3, thereby forming a magnetic circuit. A conical frame 7 is mounted on the plate 4 through screws 8a. A conical diaphragm 8 is attached to the frame 7 around an upper edge thereof. A lower edge of the diaphragm 8 is connected to a damper 6 and a voice coil 5. A magnetic gap is formed between the center pole 2 of the yoke 1 and the plate 4. The voice coil 5 is supported by the damper 6.
On the center pole 2 is mounted a plastic spacer 10 through a screw 10a. The mid-and high range speaker unit 11 which forms an electromagnetic circuit comprising a voice coil (not shown), is mounted on the spacer 10. The coaxial loudspeaker is advantageous in that the construction thereof is simple, and that, since both of the speaker units 9 and 11 have the same sound source location, the sound image is stable.
However, in the loudspeaker employing the spacer, sound pressure levels increases in a mid-frequency range, specifically in a range between 1 and 5 KHz, thereby decreasing the sound quality.
More particularly, as shown in the graph of FIG. 18, the sound pressure levels of the speaker with the spacer, which is indicated by a solid line, increases at frequencies about 2 KHz, compared to that of a speaker without a spacer, which is shown by a dotted line. The cause of the increase is that the sound pressure increases in a space between the spacer 10 and the diaphragm 8 of the woofer 9, thereby raising the acoustic impedance thereof.
In order to solve the problem, the height of the spacer 10 is increased, and the outer diameter thereof is decreased as much as possible. However, the acoustic characteristics of the loudspeaker cannot be sufficiently improved just by adjusting the shape and the position of the spacer 10.